Use of static mixing element in connection with flow of gas and liquids through a production tubing

ABSTRACT

The present invention relates to a method and a device in connection with a method for transport or flow of gas and liquid in a pipe, by which method there is in the overall or part of the pipe mounted static mixing elements for achieving favorable flows in the pipe, and for the purpose of being able to stimulate petroleum wells which have terminated production or almost terminated production due to high pressure head resistance, it is according to the invention suggested that the pipe which is used, is a production tubing or production string extending substantially vertically from a production reservoir having a low pressure, and that the mixing elements residing in the tubing is arranged in such a manner that the gas bubbles in the oil are distributed optimally as carriers in the oil, for thereby reducing and evening out the pressure head resistance in the tubing.

FIELD OF THE INVENTION

The following invention relates to a use in connection with transport orflow through a production tubing string in a petroleum well of gas andliquid from a petroleum reservoir with low pressure, where the flowingfluid comprises a substantial liquid part and a less gas contribution,which gas mostly flows or bubbles past or through the liquid in theproduction tubing.

BACKGROUND OF THE INVENTION

In other words, it can be said that the present invention relates mainlyto the type of petroleum wells which may be termed an oil producer, suchoil producer producing mainly oil and some gas. This is different frompetroleum wells (reservoirs) which are termed gas producers, which gasproducers often also produce some liquid, for example in the form ofcondensate and water. By such a liquid production (in a gas well) andlow pressure in the reservoir, such a well could, however, gradually“drown”, and the need for gas lift may be required. Thus, the presentinvention also relates to gas lift for gas wells.

Most petroleum producers (wells) will have a characterization somewhatbetween these extremes (liquid with some gas—gas with some liquid), theliquid parts here being considered as having reasonably low viscosity.

When an oil producer (with some gas included) gradually looses itsproductivity by falling reservoir pressure (and lowest practicalreceiving pressure above ground), the pressure head in the productiontubing will gradually balance the reservoir pressure. Production willthen stop and the liquid level in the well will gradually come to restbelow ground level.

Gas which is still released from the oil production tubing and in thezone close to the perforation, will bubble up through the productionstring, and then without effectively contributing to the reduction ofpressure head.

It is in connection with this type of wells that the present inventionhas been developed.

By such oil producing wells also the residual gas will gradually beterminated, and in order to prevent such wells from “dying” it is commonto introduce gas lift to reduce the pressure head resistance.

However, the present invention is to the fact of utilizing thepreviously ineffective gas bubbles flowing through or bubbling past theliquid in a production tubing string, and then in a surprising and newmanner.

PRIOR ART

From prior art there should be mentioned applicant's NO-C-146.826relating to the evening out of multi-phase flow by means of staticmixing elements.

Further, there are known static mixing devices in a well fromCA-C-1.300.002 and U.S. Pat. No. 4.832.127, but in these cases steam andwater, respectively, are introduced in order to improve the wellproductivity.

COMPARISON WITH PRIOR ART

It is previously known from applicant's NO patent 146.826 to use staticmixing elements in rising pipes, but then in connection with a differenttype of application, namely a typical petroleum platform riser pipe, inwhich slugging flow may occur. The static mixing elements are thenmounted in order to even out or prevent the development of such flowslugging, and the used mixing elements must in this connection beregarded as energy damping or friction stimulating elements, whichcontributes in dampening the excessive or pulsing energy giving rise tosaid “slugging”. In other words, the flow pattern is transferred from“slugging” to “mist flow” due to the changed hydraulic conditionsintroduced by the mixing elements.

In contrary to such a use in order to avoid “slugging”, one has inconnection with oil producing reservoir found that even if the mixingelements in their basic design are to be regarded as friction elements,they can surprisingly be used as liquid lifters for petroleum wellswhich produce substantially only liquid, which in turn is too heavy tolift itself.

Without such mixing elements the few gas bubbles which appear in theproduction tubing string, will only be greater and greater the furtherup they rise, which is to the fact that they become more and moreineffective as regards their lifting contribution. By using staticmixing elements in that type of production reservoir which is the casehere, it has surprisingly been found that the otherwise by-bubbling orthrough-bubbling gas will be finely distributed with such a fineness inthe dominating liquid volume, that there is obtained a lower pressurehead and thereby a stimulating lifting effect for the resultingliquid/gas-mixture. Consequently, according to the invention, aninstruction of how the gas bubble energy can be channelled or convertedinto an energy stimulating result is obtained.

Likewise, by introducing, according to the invention, a string of mixingelements in the production piping string, the need for gas lift will bereduced because there is achieved a lower mean pressure head, and thenat a lower total gas flow rate. By only introducing a string having suchmixing elements in a production well of the stated art, before the wellis completely dead, it will thus be possible to increase the lifetime ofthe well until, sooner or later, it is necessary to introduce additionalgas lift. For gas wells having low pressure and liquid production,wherein gas lift is installed, the requirement for lifting gas will alsobe reduced by the introduction of a string of mixing elements.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides not only an instruction for a specificfield of application, but also the unexpected utilization of “useless”large bubbles in the form of distributed “small bubble carriers”. Thisinvolves providing the liquid and the gas to have an effective meandensity which in character tends towards a single-phase flow.

It is important to recognize in the present case, that the introductionof mixing elements in a production tubing, wherein productivity issatisfactory will not contribute to a further improvement of theproductivity, but rather a reduction due to the increased fall inpressure. Nevertheless, one has in connection with the present inventionsurprisingly found that the introduction of the mixing elements in awell at falling reservoir pressure, wherein the pressure head in theproduction tubing gradually evens out the reservoir pressure, willinvolve the exact opposite result, i.e. such “reduction elements” willoperate in a completely opposite manner, namely to reduce the pressurehead resistance and stimulate lifting effect for the liquid dominatingliquid/gas-mixture.

Thus, especially when the production is unable to operate alone and theflow is low and the pressure head is low, the use of static mixingelements will give a positive stimulating effect by the distribution ofgas in liquid.

Further features and advantages of the present invention will appearfrom the following description taken in conjunction with the appendingdrawings, as well as from the further attached patent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematical view illustrating the principle of the presentinvention, especially used in a production tubing string in a petroleumwell, including an enlarged cut-out of the production tubing string.

FIG. 2 is a schematical view illustrating a further embodiment of theinvention.

FIG. 3 is a schematical view illustrating the principle according to theinvention applied in connection with further auxiliary devices.

DETAILED DESCRIPTION OF EMBODIMENTS

The invention can be used in a petroleum well with a production tubingstring as illustrated in the attached FIG. 1.

The object of the invention is to stimulate petroleum wells of the typewhich has terminated—or almost terminated the production, by creating anoptimum gas in liquid distribution and an optimum profile of the averagegas/liquid density along the extent of the production tubing string.This in order to reduce the pressure head resistance at the bottom ofthe string as regards the flow of gas/liquid out of the reservoir.

The type of flow patterns which are found in typical production tubingsin petroleum wells, is dependent upon gas/oil conditions and thepressure in the reservoir. The invention can be utilized in petroleumwells wherein the liquid portion is decreasing or terminates its flow bylow reservoir head. By low head in the reservoir and a greatcontribution of liquid in the flow from the reservoir to the productiontubing, it will be possible that the liquid flow in the productiontubing is reduced or stopped because the pressure head of the flow hasbecome relatively too large. This because the gas to a large degreeflows or bubbles past the liquid in the production tubing. In otherwords, by an unfavorably mixed gas and liquid (gas bubbling through theliquid) the pressure head resistance will strive towards the liquidcolumn. By mixing the liquid and the gas in the production tubing it ispossible to achieve an effective average density and a correspondinglylower pressure head resistance. In other words, by a favorable mixtureof gas and liquid the mixture will have a character of striving towardsa single-phase having a mean density and correspondingly lower pressurehead resistance. It is this effect which is to stimulate the well flowfrom oil reservoirs having low pressure.

The stimulation of dying wells, or wells having a low reservoirpressure, is currently being done by stimulating the reservoir itself inorder to increase the pressure at the bottom of the production tubing orby devices in the production tubing which help the liquid upwardly inthe production tubing. If the stimulation of the reservoir itself toincrease the pressure at the bottom of the production tubing, is notpractically possibly or effective, such devices in the production tubinghelping the liquid upwardly in the production tubing, will normallyrepresent the selected method. Methods having widespread applicationcomprises various downhole pumping devices and gas lift systems.

The use in accordance with the invention is to the fact that staticmixing elements, here specifically designated as N1-NN, are mounted inpipe lengths 1 of the overall or parts of the production tubing string3, sea the example in FIG. 1.

The elements N1-NN causes a mixing of the gas and the liquid phases, andthen by means of a plurality of splittings and mergings along the extentof the pipe string. The use, which is here disclosed, is in principlenot dependent upon a specific type of mixing elements, the use beingsubstantially to the fact of improving the gas in liquid distributionand to create an optimum profile of the average gas/liquid density alongthe extent along the production tubing by the right choice of mixer“fineness”.

In FIG. 1 it also appears that each tubing length of the productiontubing string 3 appropriately can have mounted therein said staticmixing elements N1-NN, which tubing lengths 1 appropriately can bescrewed together through fittings 2 as a production tubing string 3 inthe well in question.

Further, the embodiment of FIG. 1 illustrates that through the mountedmixing elements N1-NN there are provided a central guiding 5, to whichguiding there via an appropriate upper valve 4, can be conductedpossible gas for gas lifting. The guiding 5 can be made as a part of themixing element connection.

In FIG. 1 there is also illustrated an oil zone, or an oil reservoir 6,from which gas and liquid 7 will flow through appropriate perforations 8to said production tubing string 3 and upwards towards the well head 9.where gas and liquid via appropriate valves are passed on to a separatorthrough a separator line 10.

It is previously known through Norwegian patent number 146.826, bearingthe title “Method and equipment for evening out a rising multi-phaseflow in slanting and vertical pipes”, that static mixing elements can bemounted in rising pipes in order to level out, or prevent the creationof liquid “slugging”. The use of the invention therein, should be alevelling out of the multi-phase flow in rising pipes (platform risers)wherein the flow rate was given, and was too low to avoid “slugging”.The previously known technique had not for its objection to create anincrease in the total flow in the rising pipe.

A use of static mixing elements mounted in the overall or parts ofpetroleum production tubing, as this is suggested according to thepresent invention, thus represents a new application in relation topatent number 146.826, because the objection is now to increase thetotal flow in dying petroleum wells. Consequently, this novelapplication is primarily some sort of gas lift without introduction ofgas.

However, it is to be understood that the invention can be used togetherwith gas lift in order to reduce the need for lifting gas. This forthose cases wherein a further increase of the production is wanted orpossible.

In FIG. 2 there is illustrated a second example of a petroleum well,wherein the present invention can find its application, and wherein gaslift can also be used.

Possible lifting gas will here be supplied to the production tubing 103via a tubing 105 which runs on the outside of the production tubing 103,said lifting gas being controlled by means of an appropriate upper valve104.

The gas can appropriately be introduced in the production tubing 103just above the perforations 108 towards the oil reservoir 106.

FIG. 3 illustrates further example of the invention used in connectionwith further auxiliary devices.

Also here there is from an oil reservoir 206 supplied gas and liquid tothe production tubing string 203 in a petroleum well, said gas andliquid being transported via the well head 209 to a separator line 210.By means of said auxiliary devices in FIG. 3 there will in the separatorline 210 be created a moderate sub-pressure or vacuum, which togetherwith the use of said discussed mixing elements N1-NN in connection withFIG. 1, will further contribute to the liquid lifting effect forpetroleum wells given by the gas bubbles in the liquid containing littlegas.

Even moderate vacuum will create a “Champagne” effect in the wells inquestion, which can be utilized to a maximum by the mixing elementsN1-NN in order to reduce the pressure head resistance.

FIG. 3 illustrates also that said separator line 210 is connected withan ejector 211 which, in turn, is connected to a separator tank 212,wherein is created a small over-pressure. At the bottom of the separatortank 212 there is connected a pump 213 which from the tank 212 pumps outproduced oil through an oil level valve 214, which, in turn, isconnected with the tank 212 via an appropriate level meter 215.

Between the pump 213 and the oil level valve 214 there is via an ejectorcontrol valve 216 connected a feedback line 219 which, in turn, isconnected to said ejector 211 for control of said moderate sub-pressureat the well head 209.

From the top of the separator tank 212 there will via a gas valve 217 betapped off produced gas, under the surveillance of a pressure meter 218.

What is claimed is:
 1. A method for transport or flow of gas and liquidthrough a production tubing string in petroleum wells from a productionreservoir having low pressure, said method comprising the steps ofpassing a mixture of liquid and a gas which flow through or bubbles bythe liquid in the production tubing string; and arranging static mixingelements in the production tubing string to finely distribute the gas inthe liquid to achieve a lower pressure head resistance and thereby astimulating lifting effect for the resulting liquid/gas mixture.
 2. Themethod as stated in claim 1, and further comprising the step ofproviding the liquid and the gas with an effective mean density whichtends towards single-phase flow.
 3. The method as stated in claim 1, andfurther comprising the steps of pre-mounting the static mixing elementsin said production tubing string elements and individually removing saidstatic mixing elements for replacement or maintenance.
 4. The method asstated in claim 1, and further comprising the step of providing saidstatic mixing elements either alone or in connection with means forsupply of lifting gas.
 5. The method as stated in claim 1, and furthercomprising the step of supplying the lifting gas in a pipe which iscarried on the outside of the production tubing string.
 6. The method asstated in claim 1, and further comprising the step of connecting anejector for vacuum operation of a well head and a moderately pressurizedreceiving separator at the top of the production tubing string.
 7. Themethod as stated in claim 2, and further comprising the steps ofpre-mounting the static mixing elements in said production tubing stringelements and individually removing said static mixing elements forreplacement or maintenance.
 8. The method as stated in claim 2, andfurther comprising the step of providing said static mixing elementseither alone or in connection with means for supply of lifting gas. 9.The method as stated in claim 2, and further comprising the step ofsupplying the lifting gas in a pipe which is carried on the outside ofthe production tubing string.
 10. The method as stated in claim 2, andfurther comprising the step of connecting an ejector for vacuumoperation of a the well head and a moderately pressurized receivingseparator at the top of the production tubing string.
 11. The method asstated in claim 3, and further comprising the step of providing saidstatic mixing elements either alone or in connection with means forsupply of lifting gas.
 12. The method as stated in claim 3, and furthercomprising the step of supplying the lifting gas in a pipe which iscarried on the outside of the production tubing string.
 13. The methodas stated in claim 3, and further comprising the step of connecting anejector for vacuum operation of a well head and a moderately pressurizedreceiving separator at the top of the production tubing string.
 14. Themethod as stated in claim 4, and further comprising the step ofsupplying the lifting gas in a pipe which is carried on the outside ofthe production tubing string.
 15. The method as stated in claim 4, andfurther comprising the step of connecting an ejector for vacuumoperation of a well head and a moderately pressurized receivingseparator at the top of the production tubing string.
 16. The method asstated in claim 5, and further comprising the step of supplying thelifting gas in a pipe which is carried on the outside of the productiontubing string.